This invention relates to microwave diplexers, and in particular to a diplexer arrangement having high isolation between the transmit and receive ports when the transmit/receive frequency separation is small.
A diplexer is a combination of two bandpass filters having two separate transmit/receive ports and a common port. Isolation between the transmit and receive ports is required in order to isolate the relatively high power transmit signal from the relatively low power received signal. This isolation is measured at the passband of the filters and typically exceeds 80 dB. Diplexers are either fixed tuned or tunable over a range of transmit/receive frequencies by tuning the filter's resonators and adjusting, if necessary, its couplings. When a signal is applied to the transmitter port of the diplexer, it propagates through the transmit bandpass filter and reaches the common port. There, the adjacent receive bandpass filter, which is tuned to a lower or higher frequency, produces a very high impedance and hence the transmit signal passes through the common port where it sees a matched load. A very small amount of signal energy passing through the adjacent receive filter is attenuated by the receive bandpass filter's stop band attenuation. Hence, the isolation is a function of filter selectivity.
Bandpass filters provide attenuation for signals at frequencies outside the filter passband by reflection. The reflection of signals is caused by a mismatch condition provided by the filter. This mismatch condition increases towards frequencies away from the passband. Mismatch is a function of the impedance seen at the input of a filter. If the impedance vs frequency exhibits a singularity (a pole or a zero) at a certain frequency, then the transmission at that frequency will be zero total reflection, no transmission through the filter. Due to the non-ideal nature of filters, the transmission zeros actually appear as points of extremely high attenuation, instead of infinite attenuation.
Diplexers with Chebyshev bandpass filter responses are known. When the frequency separation between the transmit and receive ports is large, Chebyshev filters will provide sufficient selectivity and are easy to realise. However, with Chebyshev filters, stop band attenuation increases monotonically and therefore cannot be used for very small transmit/receive frequency separation, where sharp selectivities are required. To provide a practical diplexer that has very small transmit/receive frequency separation, two highly selective bandpass filters are necessary. To fulfil this requirement, it is mandatory to use bandpass filters with transmission zeros.
Combline filters with transmission zeros, created by couplings between non5 adjacent resonators are known and have been used in single filters, but are not commonly used in tunable diplexers because the required adjustability of the transmission zeros over the tuning frequency range of the diplexer is too difficult to achieve. Diplexers require that the correct location of the transmission zeros, relative to the filter's centre frequency, be maintained for each centre frequency within the diplexer's tuning range in order to provide the required isolation between the transmit and receive ports.
The difficulty in achieving adjustable transmission zeros in a diplexer having two combline filters is, that in order to create any desired transmission zeros above the pass band of one filter and below the passband of the other filter, one filter must include adjustable inductive cross-couplings between non-adjacent resonators, and the other filter must have adjustable capacitive cross-couplings between non-adjacent resonators. The filter containing inductive cross-couplings will have its transmission zeros above its passband, and the filter containing capacitive cross-couplings will have transmission zeros below its passband.
In the filter having inductive cross-couplings, due to the fact that one resonator may be common to both inductively cross-coupled sections, excessive interaction between these cross-couplings may occur and as a result one transmission zero may not be produced. Further, the inductive cross-couplings would normally be provided by wire loops, and the magnitude of cross-coupling provided by the loops is difficult to adjust.
Co-existence of the two capacitive cross-couplings with one resonator being common to both cross-couplings constitutes yet another problem, if independent adjustment is required.